"Over time, many people have complained about the X Window system; the X Window system, or Xorg in its current most popular implementation, is the layer between applications and the graphics adapter. It has some fantastic features (like the ability to run application over the network) and some shortcoming.
One thing is sure: it has evolved over the last year or so, immensely, especially as far as 3D and hardware acceleration."

the most interesting use of XCB, compared to Xlib is for windows managers. The only thing you will gain for toolkits using XCB is the startup which can be faster if XCB is correctly used (and a lot faster if the client is launched over a ssh pipe). You can gain a bit more if the main loop that the toolkit and the windows manager use are 'compatible' or the same, so that round trips are (almost) all removed.